Add solver failure reporting to cabal-install

[lambda-fairy]

This patch adds build reporting for dependency solver failures. It is motivated by Hackage, where such failures would leave no indication of what went wrong.

To simplify implementation, only targets with an exact name and version are reported. This includes e.g. hello-1.0, but not hello > 1.0 or hello. This is good enough for Hackage, as we specify packages exactly anyway.

The feature is guarded behind a flag (--report-planning-failure), as most users don't need it.

[tibbe]

Could you give an example of what this would output and how it's different from running e.g. cabal install -v3? Thanks!

[lambda-fairy]

@tibbe Sure!

If we give cabal-install an impossible set of constraints, such as:

cabal install hello-1.0 --constraint='base < 4' --constraint='base > 4' --build-summary=summary.txt --report-planning-failure

Then summary.txt will be populated with the following:

package: hello-1.0
os: linux
arch: x86_64
compiler: ghc-7.6.3
client: cabal-install-1.21.0.0
install-outcome: PlanningFailed
docs-outcome: NotTried
tests-outcome: NotTried

I'm not sure what -v3 has to do with this patch, as it doesn't affect what is written to the console.

[tibbe]

👍

@dcoutts does this make sense to you?

[lambda-fairy]

I'm taking a different approach to this, so closing for now.